1. Field of the Invention
The invention relates to structural materials and methods used in the general areas of quality and nondestructive testing, structural health monitoring, and detection of forces.
2. Description of the Related Art
Ultrasonic testing and vibrational resonance techniques are commonly used nondestructive techniques to test the integrity of polymer matrix composite structures and other structures. These techniques may be used to perform quality control tests at the time of manufacture, and/or structural health monitoring tests over the life of the structure. Piezoelectric transducers have been placed on such materials to both generate and receive ultrasonic and vibrational signals.
Ultrasonic testing techniques may employ longitudinal, shear, or surface waves. Defects existing in the structure may cause an irregularity in the ultrasonic signal, if the defect is located in an area probed by the ultrasonic signal. Defects in the structure may also alter its vibrational resonance response, if the defects are located in an area that is probed by the vibrational signal introduced into the structure.
For nondestructive quality control testing, skilled and expensive technicians have been needed to properly perform ultrasonic scans. Such ultrasonic testing is often a time consuming operation that slows down production. Vibrational resonance techniques have not been as commonly used, due to the expense and complexity of developing the necessary experimental apparatus. The ultrasonic method in particular suffers from the drawback that it is directional. Thus only limited parts of the structure may be examined at any one time. Full characterization of a structure, especially a complex structure, is a difficult, time-consuming, and expensive operation.
In large structures, such as those used on aircraft and bridges, piezoelectric wafer active sensors have been utilized. Piezoelectric wafer active sensors are flat, rectangular sensors made of ceramic material. The sensors are bonded to the structure, and their response over time is examined as part of a structural health monitoring operation. One problem with piezoelectric wafer active sensors is that they must be bonded to the structure, and the bond is prone to break down over time. Another problem is that such sensors do not easily conform to curved surfaces, and are easily broken. They may also produce unacceptable irregularities on aerodynamic surfaces, or they may detract from the aesthetic appeal of other structures.
From the foregoing it will be appreciated that improvements of this area of endeavor would be welcome.